1. Field of the Invention
The present invention relates to a method and apparatus for detecting timing pulses for a scrambled television system and, more particularly, to a method and circuit arrangement for detecting timing pulses for descrambling a scrambled television signal, which is transmitted to subscribers through a CATV (Cable Television) system, a civil satellite television broadcasting system, or the like, in order to permit only the contracting subscribers to receive the scrambled television signal in normal condition.
2. Description of the Prior Art
In a CATV system and a satellite broadcasting system, it is common to scramble a television signal in order to disturb television pictures, so that only the contracting television subscribers are able to receive television programs by implementing a proper descrambling operation at the receiving end.
In case of a CATV system, to scramble the television signal, the synchronizing signal of the television signal is suppressed at an interval, for example, of every fifteen-frames with different rates of suppression for asynchronizing the television system at the receiving end.
As shown in FIGS. 1A-1D, in a CATV center at the transmitting end, a base band television signal (FIG. 1A) is converted into a television intermediate frequency (IF) signal (FIG. 1B), horizontal synchronizing intervals and vertical blanking intervals (VBI) including blanking intervals of a video signal are suppression at the same rate of suppression (FIG. 1C). The obtained television signal is in turn sent out after multiplexing timing pulses (TP), which include data relating to the rate of compression, on a frequency modulated (FM) sound IF signal through amplitude modulation (AM) in synchrony with a positive going edge of each horizontal synchronizing interval of the television signal (FIG. 1D).
The compression data are decoded in a CATV terminal unit at the receiving end based on the received timing pulses, whereby stretching the horizontal synchronizing intervals of the received television signal, which was compressed at the transmitting end, by utilizing amplitude stretching data obtained from the decoded compression data for providing normal pictures on a display screen for viewing.
Now referring to FIG. 2, there is shown a block diagram illustrating the operation of the CATV terminal unit, wherein the scrambled IF signal amplified at an IF amplifier circuit 20 is filtered by a band pass filter 21, which consists of a SAW (surface acoustic wave) filter, for emphasizing a video carrier wave and sound carrier wave, and then an FM inter-carrier signal of 4.5 MHz sound signal is derived from a video detector circuit 22.
The FM sound signal is in turn filtered by a 4.5 MHz.+-.250 KHz band pass filter BPF 23, and fed to an AM detector circuit 24 to produce the timing pulses (TP) which have been multiplexed on the FM sound signal by amplitude modulation at the transmitting end. The timing pulses (TP) thus produce are then fed to a descrambler circuit 25.
At the descrambler circuit 25, data relating to the rate of suppression contained in the timing pulses (TP) is detected and then data relating to a rate of restoration is generated in such a timing as being synchronized with the timing pulses (TP), thus restoring the horizontal synchronizing intervals and vertical blanking intervals, which have been suppressed at the transmitting end, to that of the unscrambled television IF signal by restoring them to respective amplitudes in accordance with the rate of restoration as in U.S. Pat. No. 4,567,517.
However, in accordance with the aforementioned inter-carrier system, the mixing of a component part of a video signal with the sound signal is unavoidable, and there have been such problems that the timing pulses (TP) are hardly detected in a produced beat signal of 4.5 MHz due to mutual interference of the modulated video and sound signals contained therein.
Further, in recent years, there are several different types of broadcasting systems such as a bilingual broadcasting, stereophonic broadcasting and the like wherein an auxiliary sound signal carrier wave F.sub.N is located in close range (approximately 224-350 KHz) to a main sound signal carrier wave F.sub.A by multiplexing. These broadcasting system also have problems such that, in accordance with a simple inter-carrier system, the video signal carrier wave F.sub.V, the main sound signal carrier wave F.sub.A and the auxiliary sound signal carrier wave F.sub.N may cause interferences with each other and this makes hard to detect the timing pulses correctly. It is also difficult to separate the main sound signal carrier wave F.sub.A from the auxiliary sound signal carrier wave F.sub.N as they are located so close to each other in frequency.
It is therefore an object of this invention to provide a method and apparatus for detecting timing pulses for use in descrambling, wherein detected are the timing pulses, with high accuracy, contained in a scrambled television signal as multiplexed on a main sound signal through an amplitude modulation.